Ethylene is the keystone of the petrochemical industry, since; it is employed as the main building block for the production of polymers, ethyl-benzene and styrene, among other chemical products of great importance in the modern world. Ethylene is produced from the steam-cracking (pyrolysis) of saturated hydrocarbon cuts, chiefly ethane and propane. Such processes are carried out in the presence of superheated steam at temperatures within the range 800-1000° C. Operating under these conditions involves a huge energetic demand and very high expenses related to the cost and maintenance of the furnaces which provide the heat required for the process. Also, due to the usage of high temperature, a wide variety of byproducts are formed, such as di-olefins as well as acetylene being the ones observed at the largest concentrations. The separation of these formed species from the reactor effluent requires a relatively complex scheme involving extractive distillation and/or selective hydrogenation, the latter in the particular case of having acetylene, which at the end requires of an additional investment. As a consequence, for economic and environmental reasons, several worldwide companies and research groups have focused their efforts on finding alternative process to produce ethylene.
An attractive route to produce ethylene is through the oxidative dehydrogenation reaction of ethane. The oxidative dehydrogenation of ethane (ODH-E) is an exothermal reaction which is not limited by the thermodynamic equilibrium and, hence, full ethane conversion is possible at low reaction temperatures (<500° C.). In the ODH-E, additionally, the number of side reactions is rather limited; usually, carbon monoxide and carbon dioxide appear as the main side products, while the formation of coke is negligible. Despite the many efforts dedicated to obtain catalysts with high activity and selectivity features, to the date, industrial application of ODH-E is still far from a reality. In fact, economic calculations have indicated that ethylene yields reported so far during ODH-E are not yet sufficient to be considered as an economically profitable process. It is therefore clear that more efforts are required to further improve the catalyst performance whilst, in the process context, particular attention is to be paid to design an adequate reactor configuration due to the thermal characteristics of the reactions involved.
Vanadium based catalysts supported on conventional materials were the first catalytic systems used for the ODH-E, notwithstanding, their efficiency to produce ethylene was not very high (Oxidative dehydrogenation of ethane and propane: commercial How far from implementation? Cavani et al., Catalysis Today, 127 (2007) 113). In particular, at high ethane conversions, an important amount of carbon oxides and acetic acid were observed in detriment to ethylene formation.
The use of catalysts based on oxides of molybdenum and vanadium together with other oxides of transition metals, e.g., Ti, Cr, Mn, Fe, Co, Ni, Nb, Ta or Ce, calcined at 400° C., was proposed by Thorsteinson et al. in “The Oxidative Dehydrogenation of Ethane over Catalyst Containing Mixed Oxides of Molybdenum and Vanadium”, Journal of Catalysis, 52 (1978) 116. The best result was obtained over a solid with the composition Mo0.61V0.31Nb0.08 supported in gamma alumina, yielding a 25% of ethylene at 340° C.
Later, in U.S. Pat. Nos. 4,250,346, 4,524,236 and 4,568,790 assigned to Union Carbide Corporation, the synthesis of catalyst for ODH-E at low temperature is reported. U.S. Pat. No. 4,524,236, in particular, discloses a catalyst with a composition MoVNbSbM (M being at least one of the following elements Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U and W). The best catalytic result offered in this patent was obtained over the Mo0.61V0.26Nb0.07Sb0.04Ca0.02 system, exhibiting an ethane conversion equal to 34% and selectivity to ethylene of 86% when the reaction was conducted at 330° C. After a further increase in the reaction temperature to 400° C., 73% of the fed ethane was converted with an ethylene selectivity of 71%. In reference U.S. Pat. No. 4,250,346, the formation of acetic acid is reported to occur during the ODH-E.
In U.S. Pat. No. 5,162,578, granted to Union Carbide Chemicals & Plastics and Union Carbide Corporation, and EP 0294846A3 with Union Carbide Corporation as applicant, McCain and co-workers claim about a catalytic composition with a general formula MoaVvNbSbXe (X being at least one of the following metals Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U and W, preferably Ca) to produce acetic acid from ethane, or a mixture ethane/ethylene, with a remarkably high selectivity to the mentioned acid.
Japanese patent JP 10143314 granted to Mitsubishi Chemical Industries Ltd. describes a MoVSbX catalytic system (wherein X corresponds to Ti, Zr, Nb, Ta, Cr, W, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Zn, In, Sn, Pb, Bi, Ce and alkaline rare earth metals) which exhibits a crystalline structure defined by a X-ray spectrum included in the patent. The catalytic system was used for the selective oxidation of ethane to ethylene with ethane conversions as high as 90.8% and selectivity to ethylene of 68%.
MoVNbSb mixed oxides have been also proposed as effective catalysts for the oxidative dehydrogenation of ethane to produce ethylene, as well as acetic acid, in patent EP-A-0294845 granted to Union Carbide.
WO 99/13980, assigned to Saudi Basic Ind., in turn, reports a Mo—V-Nb based catalyst doped with small amounts of P, Hf, Te and/or As. Solids were thermally treated under air atmosphere between 250 and 450° C. and then used in the oxidation of ethane to produce acetic acid, reporting yields within the 12-17% range.
In Japanese Patent JP10017523 granted to Mitsubishi Chemical Industries Ltd. in 1998, inventors proposed a catalyst for the oxidation of ethane to acetic acid. It is composed of a metal—Ru, Rh, Jr Pd and/or Pt—supported over a mixed oxide of MoVXZ. In this patent, particular attention is paid on a Pd based catalyst supported on a mixed oxide constituted of MoVNbSb, which exhibited yields to acetic acid as high as 59.7%.
Several US patents, in particular U.S. Pat. Nos. 6,030,920A, 6,194,610B1, 6,310,241B1, 6,383,977B1 and U.S. 2003/0100794A1 have been granted to Saudi Basic Industries Corporation, a Saudi Arabian company. These patents report on the performance of Mo and V based catalysts promoted with other metals for the oxidative dehydrogenation of ethane at low temperatures. The catalytic activity experiments contained in these documents, however, were carried out using molecular oxygen as an oxidant to yield mainly acetic acid, while ethylene is formed as byproduct.
In U.S. 2003/01000794A1, a new catalyst with a general formula MOaVbAlcXdYeOz, where in X is at least one of the elements belonging to the group W and Mn; Y is at least one element selected from the group Pd, Sb, Ca, P, Ga, Ge, Si, Mg, Nb and K; an “z” is an integer number representing the number of oxygen atoms required to satisfy the valence of Mo, V, Al, X and Y. These catalysts were utilized in the partial oxidation of ethane to produce acetic acid as well as ethylene.
Additionally, methods to produce catalyst containing Mo, V, Sb and Nb are also claimed in U.S. Pat. Nos. 6,610,629 B2 and 7,109,144 B2, both assigned to Asahi Kasei Kabushiki Kaisha. The composition of the catalyst is represented by the general formula Mo1.0VaSbbNbcZdOn. In the latter, Z corresponds to at least an element belonging to the group W, Cr, Ti, Al, Ta, Zr, Hf, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Zn, B, In, Ge, Sn, Pb, Bi, Y, Ga, rare earths and alkaline rare earth metals. These catalysts were employed in the ammoxidation of propane or isobutene.
In U.S. 2008/0161602A1, which claims the benefits from the provisional application patent U.S. Ser. No. 60/877,270, describes a catalytic formulation denoted by the general formula MoaVbNbcTedSbeOf, wherein a=1, b=0.01-1.0, c=0.1-1.0, d=0.1-1.0 e=0.01-1.0 and f depend upon the oxidation state of the other elements. A particular feature of these catalysts is to exhibit at least two crystalline phases, namely, an orthorhombic one denoted as M1 and a second pseudo-hexagonal phase named M2. The referred solid is used to promote the partial oxidation of propane to acrylic acid, acetic acid being one of the most important side-product.
U.S. 2011/0245571A1 and U.S. 2012/0016171A1, to Nova Chemicals International S.A., claim a process for the preparation of a catalyst for the oxidative dehydrogenation of ethane, with a relatively high yield to ethylene reporting selectivity to ethylene higher than 90% with productivity in the range 2,500 g ethylene per hour and kg of catalyst. The catalyst claimed is a tellurium-containing solid with as general formula VxMoyNbzTemMenOp, wherein Me is a metal belonging to the group Ta, Ti, W, Hf, Zr and Sb, or a mixture of them. Metals are deposited over a matrix composed of oxides of Ti, Zr, Al, Mg, La, Si or mixtures thereof, or even a matrix of carbon.
U.S. 2010/0256432A1, assigned to Lummus Novolent GMBH/Lummus Technology Inc., and U.S. Pat. No. 8,105,971 B2 to Lummus Technology Inc., claim a high performance catalyst for the oxidative dehydrogenation of ethane to ethylene. Over this catalytic system represented by Mo1.0V0.29Nb0.17Sb0.01Te0.125Ox, ethane conversion reached values of up to 81% with an ethylene selectivity of 89% when reaction is conducted at 360° C. Notice that this solid also contains tellurium as an ingredient of the formulation.
U.S. 2006/0183941A1, J. L. Dubois, W. Ueda et al., in contrast, claims a tellurium-free catalyst represented by the general formula Mo1.0VaSbbNbcSidOx, in which a=0.006-1.0, b=0.006-1.0, c=0.006-1.0, d=0-3.5 and “x” is the amount of oxygen bonded to other elements. The catalyst is applied to the partial oxidation of propane to yield acrylic acid.
One of the most efficient catalysts for the oxidative dehydrogenation of ethane to ethylene has been described in WO 03/064035 and U.S. Pat. No. 7,319,179, by J. M. Lopez-Nieto et al., and granted to UPV-CSIC. There is claimed a catalyst based on a mixture of mixed oxides, MoTeVNb, which exhibited a yield to ethylene close to 75%. This high-efficiency solid contains tellurium as well.
Similar catalysts have been reported in the open literature. Ueda et al. in “Selective oxidation of light alkanes over hydrothermally synthesized Mo—V-M-O (M=Al, Ga, Bi, Sb, and Te) oxide catalysts”, Applied Catalysis A: General 200 (2000) 135. The formation of acetic acid is, however, always observed in the reactor effluent. Selectivity to ethylene is lower than 75% for an ethane conversion lower than 20%.
Also, Botella et al. in “Selective oxidation of ethane: Developing an orthorhombic phase in Mo—V—X(X=Nb, Sb, Te) mixed oxides”, Catalysis Today 142 (2009) 272, used a MoVSb based catalyst for the ODH-E with a capacity to convert ca. 40% ethane and a selectivity to ethylene in the 90-92% range.
Due to the economic, technical and environmental advantages that the process for producing ethylene by the oxidative dehydrogenation of ethane has exhibited, the attention of research groups has been focused mainly on improving the catalyst formulation. One of the main challenges to be solved is the minimization of the formation of byproducts, in particular carbon oxides (COx). These compounds, apart from decreasing the global efficiency of the process, are produced via very exothermal reactions. Thus, catalysts with a high potential to be used at the industrial scale are expected to display selectivity to ethylene between 80 and 85% for ethane conversion in the range 50-60%. Moreover, the formation of oxygenate products, e.g., acetic acid and aldehydes, must be avoided on the referred catalytic systems as their presence would involve additional stages in the separation train or troubles in the reactor.
On the other hand, the presence of tellurium seems to be indispensable in most of the high efficiency catalytic systems reported in many patents to date for the oxidative dehydrogenation of ethane to ethylene. Notwithstanding, the relatively high susceptibility of tellurium to reducing atmospheres, together with the large amount of metal that is lost during the thermal activation stages, appears to be a restriction for a catalyst scaling-up to industrial level. This problem would be always latent in industrial practice since, during operation, the reaction mixture can be composed of ethane diluted in nitrogen, i.e., a reductive mixture which, in the presence of hot-spots would favor the reduction and further loss of tellurium with the consequent gradual decay in the catalytic properties of the solid.